Performance of capacitors

ABSTRACT

A capacitor may be configured with a dielectric laminate disposed on ordered or non-ordered structures. Materials for the dielectric laminate have high dielectric constant and reduce leakage current to increase breakdown voltage of the device. These materials may include titanium dioxide (TiO2) and silicon dioxide (SiO2). In one implementation, the capacitor may reside on a substrate. The capacitor may have structure (e.g., nano-tubes, nano-holes, etc;) disposed on the substrate having a surface area greater than the surface area of the substrate and a laminate conformally coating the structure, the laminate comprising a first layer and a second layer with materials that configure the capacitor with an energy density of at least 60 Wh/Kg.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Ser. No.62/650,318, filed on Mar. 30, 2018, and entitled “IMPROVING PERFORMANCEOF CAPACITORS.” The content of this application is incorporated byreference herein in its entirety.

BACKGROUND

Supercapacitors are devices with high power density, moderate energydensity, and long, stable life. These properties position these devicesto replace or compliment batteries to store and deliver electricalenergy for use in electrical circuits, alone or as part of hybridbattery/capacitor devices. Supercapacitors can also serve as discretecomponents in electronic circuits of all types because of their smallfootprint. However, most supercapacitors lack sufficient energy densityto work in many applications. Batteries therefore remain the mostreliable source of stored energy.

SUMMARY

The subject matter of this disclosure relates to improvements inperformance of supercapacitors. Of particular interest herein areembodiments with structure that can match or surpass batteries in suchproperties as storage, energy delivery, and discharge/recharge cycling,among others. As circuit components, the embodiments can optimize theseproperties to improve footprint and geometric conformity.

The structure may optimize stored energy E according to Equation (1)below:E=½CV ².  Equation (1)where, C is capacitance and V is voltage. The embodiments may, forexample, employ a dielectric with parameters (e.g., surface area,dielectric thickness, dielectric constant, etc.) to increase capacitance(C). These embodiments may also comprise materials that improve thedielectric breakdown strength to increase voltage (V). In this regard,some embodiments may comprise solid-state electronic components, forexample, carbon nano-tubes (CNTs) and a dielectric “nano-laminate”thatconformally coats the CNTs. Both the CNTs and the nano-laminate addressseveral key factors to improve capacitance C of the device. The CNTsincrease surface area of the embodiments within a given footprint.Material for the nano-laminate forms a stack of alternating layers withproperties (e.g., dielectric constant, energy bandgap, Gibbs freeenergy, thickness, etc.) tailored to achieve very high dielectricconstant (k) with low leakage current. These features afford the devicewith high breakdown voltage that is critical given the V² dependence ofenergy density E. The result, in turn, is a thin-film capacitor thatmarries favorable power density (of capacitors) with energy densitycomparable to battery storage in a lightweight, flexible, stable device.These properties can also improve operation and application of thecapacitors in electronic circuitry. Other embodiments are within thescope of the discussion here.

DRAWINGS

Reference is now made briefly to the accompanying drawings, in which:

FIG. 1 depicts a schematic diagram of an exemplary embodiment of acapacitor;

FIGS. 1A and 1B depict examples of structures that form the base of anelectrode comprising a conductor (A) or a non-conductor (B) with verylarge surface area for use in the capacitor of FIG. 1 . FIG. 1A showsthe electrode structures in an ordered array. FIG. 1B shows theelectrode structures in a non-ordered array;

FIGS. 1C, 1D, and 1E depict examples of structure for a laminate of thecapacitor of FIG. 1 ;

FIG. 2 depicts an energy band diagram of an example of the capacitor ofFIG. 1 ;

FIG. 3 depicts an Ellingham diagram of Gibbs free energy of materialsfor use in the capacitor of FIG. 1 ;

FIG. 4 depicts a schematic diagram of an example of the capacitor ofFIG. 1 ;

FIG. 5 depicts a plot of structural dimension relating surface area anddiameter of a structure (e.g., carbon nano-tubes) that increases thesurfaces area for the capacitor of FIG. 4 ;

FIG. 6 depicts a flow diagram of an exemplary embodiment of a method tomanufacture for use, for example, to manufacture the capacitor with anordered array of structures of FIGS. 1 and 4 ;

FIG. 7 depicts various images of an example of the fabrication of thecapacitor of FIGS. 1 and 6 showing possible template nano-holes in (a)and (b) and structure for carbon nano-tubes in (c);

FIG. 8 depicts an image of an example of an ordered array of structuresof the capacitor of FIGS. 1 and 6 , the structure comprising carbonnano-tubes that are vertically aligned and of about 240 μm in height;

FIG. 9 depicts a schematic diagram of an example of an ordered array ofcarbon nano-tubes of the capacitor of FIGS. 1 and 6 having structure ontwo-sides of a substrate of the capacitor of FIGS. 1 and 6 . Thenon-ordered electrode (FIG. 1B) may have similar device structure aswell;

FIG. 10 depicts a plan view from the top of an example of the capacitorof FIG. 1 ;

FIG. 11 depicts an elevation view for the cross-section of the exampleof FIG. 10 ; and

FIG. 12 depicts a flow diagram of an exemplary embodiment of a method tomanufacture the capacitor, for example, to manufacture the capacitorwith an ordered array of structures of FIGS. 10 and 11 .

Where applicable like reference characters designate identical orcorresponding components and units throughout the several views, whichare not to scale unless otherwise indicated. The embodiments disclosedherein may include elements that appear in one or more of the severalviews or in combinations of the several views. Moreover, methods areexemplary only and may be modified by, for example, reordering, adding,removing, and/or altering the individual stages.

DETAILED DESCRIPTION

Battery storage predominates in most applications because of the highenergy density of these devices. The energy density of lead-acidbatteries, for example, is about 1080 J/cm³. For better batteries, likelithium thionyl chloride (Li-SOCL₂) batteries, the energy density mayreach as high as 4,320 J/cm³. For circuitry, capacitors lack sufficientoperating parameter optimization and are too large and lackconformability.

Advances in capacitor technology aim to replicate the energy density ofbatteries. However, no capacitor has yet to exhibit capacitance highenough to match the energy density of batteries or have otherproperties, like breakdown voltage, necessary to replace batterystorage. Thin-film capacitors that use materials with high dielectricconstant (k) (e.g., TiO₂/ZrO₂ and Al₂O₃), for example, may only achieveareal energy densities in a range of from 60 J/cm³ to 400 J/cm³.Material combinations, like TiO₂/Al₂O₃, show promise because thedielectric constant is as high as 1000. Other materials, includingceramics like CaCu₃Ti₄O₁₂ (“CCTO”), also tend to exhibit very highdielectric constant (k), on the order of ˜10⁴ at room temperature. Butthese materials still suffer from high leakage current (or “losses”)that result in breakdown voltages far too low for the device to operateas a viable replacement for batteries or for many circuit applications.

The discussion that follows describes embodiments that achieve both highenergy density and high breakdown voltage. These embodiments exhibitaerial energy density upwards of 35,000 J/cm³, more than two orders ofmagnitude larger than the aerial energy density consistent withcapacitor devices to date. This feature makes the device usable for manyapplications such as electric automobiles, rapid-charge technologies,and energy harvesting. The proposed capacitor is also much smaller andlighter than most storage medium, like batteries. For use in circuitryor as circuit components, the proposed capacitor offers a light weight,low footprint device that may conform to the geometry around it toprovide enhanced embedded ability. Plus, with the proposed capacitorslarge surface area, improved dielectric constant, and favorablebreakdown strength may result in energy density that is higher than, atleast, lead-acid batteries.

FIG. 1 depicts a schematic diagram of an exemplary embodiment of acapacitor 100 in very general form. This embodiment is part of a storagedevice 102 that can store and discharge energy much like batteries andlike technology. The storage device 102 may have a substrate 104 thatsupports the structure of the capacitor 100. This structure may includea pair of electrodes (e.g., a first electrode 106 and a second electrode108) with a dielectric laminate 110 disposed therebetween. Thedielectric laminate 110 may comprise one or more layered “units” 112that reside between the electrodes 106, 108. The units 112 may comprisematerial layers (e.g., a first layer 114 and a second layer 116).

Broadly, the capacitor 100 is configured to store energy at densitiesthat rival batteries and related storage medium. These configurationscan maximize charge storing surface area, dielectric constant, andbreakdown strength while minimizing dielectric thickness according toparallel plate capacitor theory, shown as Equation (2) below:

$\begin{matrix}{{C = \frac{ɛ_{o}{kA}}{t}},} & {{Equation}\mspace{14mu}(2)}\end{matrix}$where, C is capacitance, k is dielectric constant, A is surface area, cois electric constant, and t is the thickness of the dielectric laminate110. The result is devices that allow effective tradeoffs between energystorage, specific capacitance, size, and operational electric currentsand voltages. The devices may manufacture at scale as part of integratedcircuits, as discrete electronic components of any size, and as largescale devices for very high energy storage and buffering applications asmight find use on vehicles or commercial energy storage.

The storage device 102 may be configured to store and discharge energy.The proposed devices may have a form factor that offers comparableenergy density at significant weight advantages over conventionalbatteries, which may weigh 50 lbs or more. Such reductions in size andweight may make electronic devices, like radios, laptops, GPS, and evennight vision goggles, much more portable and easy to transport. Notably,the storage device 102 may also scale as appropriate for use incircuitry as a circuit component or in vehicles, for large scalestorage, and other large scale application. As thin-films, the devicesare also very durable. Plus, unlike batteries and electrolytecapacitors, the devices contain no chemicals that are likely to degradeor leak-out over time. These features may prove very useful for militarypersonnel, for example, that may carry several electronic devices intothe field, often in harsh conditions or environments (includingunderwater), for long periods of time and typically without any reliableor ready access to a power source. And, the proposed thin-film deviceshave a power density to work in situations that require short, intensebursts of power, like cold engine starting. For military use, the powerdensity comports with high-fire discharge that may benefit advances inelectromagnetic rail-guns and related weapons technology, which mayoffer personnel potentially safer alternatives to gunpowder.

The substrate 104 may be configured to promote these form factors. Theseconfigurations may include hard materials (e.g., silicon wafers orglass) or flexible substrates made of metals (e.g., Cu, Al, etc.),polyimide, PEEK, conductive polyester, or like plastic or flexiblematerials. These materials are compatible with various types ofelectronics devices, including electronics that employ “hard” orinflexible substrates, solid-state or high-speed silicon electronics,hybrid flexible electronics (HFE), or for fully flexible or stretchableelectronics.

The electrodes 106, 108 may be configured to receive electric potentialthat energizes the capacitor 100. These configurations may employstructural units to maximize surface area (A) of the device. The firstelectrode 106 may comprise structural units to significantly magnify thesurface area of the electrode 106 and dielectric stack 110 at minimalcost to overall footprint of the device 102. In one implementation, theelectrode 108 will be conformal to the increased area structures and thedielectric stack 110 of electrodes 106. Examples of these structuralunits may be ordered or non-ordered, of varying sizes, includingdimensions in micrometers (“micro-scale”) or smaller includingnanometers (“nano-scale”), and may accommodate materials that comportwith manufacture techniques to scale the device 102 for differentapplications (e.g., from integrated circuits to vehicles to large scaleelectrical systems).

FIGS. 1A and 1B depict images for examples of the structural units. FIG.1A illustrates an ordered structure, which may comprise carbon nanotubes(CNTs). CNTs often have a tubular form factor that provides electrontransport or conductivity. FIG. 1B depicts irregular or non-orderedstructures. These examples may include members, for example, elongatetubules. However, these structures may lack consistent spatialarrangement with each other, often overlapping or contacting withadjacent or neighboring members to form a “mesh”-like network on thesubstrate 104. Care may be given to manufacture both ordered andnon-ordered structures to allow material layers (e.g., layers 114, 116)to form a coating of uniform thickness, which is important to optimizecapacitor performance. For example, one requirement may be that surfacearea of materials per footprint area is repeatable to ensure performanceparameters are uniform across different areas of the device 102 and fromone device 102 to another. For non-conducting structures of electrode106, a conformal coat may be disposed on the structures, for example, ofconducting material, prior to depositing the dielectric laminate 110.

The dielectric laminate 110 may be configured to allow proper energydensity of the capacitor 100. These configurations may conformally coatthe first electrode 106 at a thickness that comports with structure forthe electrodes 106. Generally, the dielectric laminate 110 may compriseany number of materials layers (e.g., layers 114, 116). This disclosurealso contemplates that the unit 112 may include any number of materiallayers, as well. It is also possible for the device 102 to benefit fromany number of the layered unit 112. For example, units 112 may be“stacked” to provide a plurality of the material layers 114, 116 on thedevice 102.

FIGS. 1C, 1D, and 1E depict additional configurations for theconstruction of the capacitor 100. In FIG. 1C, the construction mayinclude many layers (e.g., L₁, L₂ . . . L_(n)), which may be of the sameor different materials, have the same or different thickness, or havethe same or different parameters in general. FIG. 1D shows constructionthat leverages many units (e.g., S₁ . . . S_(n)), each having two layers(e.g., L₁, L₂). FIG. 1E depicts a construction that leverages many units(e.g., S₁ . . . S_(n)), where each of the units S_(n) comprises manylayers (e.g., L₁, L₂ . . . L_(n)). Selection of the number and order oflayers within the dielectric laminate 110 may rely on Maxwell-Wagnerrelaxation (to increase the dielectric constant). Materials for thelayers may rely on Gibbs free energy (to lower the leakage current).

Properties for the material layers may benefit the overallcharacteristics for the device 102. These properties include material aswell as dimensions (e.g., thickness). Notably, properties of thematerial layers may vary independently of each other, as well asindependently of other material layers, or other material layers foundin other stacked units, as well. This feature may allow forcharacteristics of the capacitor 100 to vary, effectively to fine tunethe storage or circuit device 102 in connection with its application.Some implementations may benefit from very thin individual layers thatminimize thickness of the stacked units to maximize specificcapacitance. Other implementations may leverage materials withparticularly good breakdown characteristics in layers that are thickeror thinner than other materials in the stacked unit(s) to selectivelyincrease the breakdown strength, potentially at cost to specificcapacitance.

Any material layers may be configured with materials that favorablyinfluence both the energy density and the breakdown voltage of thecapacitor 100. These configurations include dielectric materials, liketitanium dioxide (TiO₂), aluminum oxide (Al₂O₃), and silicon dioxide(SiO₂). Other materials include Si₃N₄, SiO_(2-x)N_(x), TiN,TiO_(2-x)N_(x), Ta₂O₅, SrTiO₃, ZrO₂, HfO₂, HfSiO₄, ZnSiO₄, La₂O₃, Y₂O₃,a-LaAlO₃, Pr₂O₃, Gd₂O₃, Lu₂O₃, BaTiO₃, SrO, CaO, BaO, ZnO, and MgO.Suitable materials may have properties that, when in combination, affordthe laminate 110 with appropriate high dielectric constant (k) andbreakdown voltage. Materials may also exhibit properties, like bandgap,dielectric strength, and Gibbs free energy, that promote lower leakagecurrent, as well. This feature may afford the laminate 110 with betterbreakdown voltage. Preference may be given to materials that exhibitboth high bandgap values (e.g., at or above 9 eV) and high dielectricstrength (e.g., at or above 20 MV/cm). The design may also benefit frommaterials that have similar Gibbs free energy, preferably within 2% ofeach other.

FIG. 2 depicts, schematically, an energy band representation of thecapacitor 100 to inform discussion of favorable bandgap values. Thelayers 114, 116 may operate as a barrier 118 and a semi-conductor 120,respectively. The semi-conductor 120 may comprise material having a highdielectric constant (k), like TiO₂. Materials for the barrier 118control leakage current. However, thickness of the laminate 110 must becarefully controlled because, as layers 114, 116 become thinner, theybecome less insulating mainly due to tunneling that allows charge“hopping” between the semi-conductor 120 in adjacent units 112. Thisphenomenon may significantly increase leakage current. Probability oftunneling, for example, may be determined in accordance with Equation(3) below:

$\begin{matrix}{{{{Tunneling}\mspace{14mu}{Probability}} \approx {\frac{16{E\left( {U_{0} - E} \right)}}{U_{0\;}^{2}}\exp\left\{ {{- 2}W\sqrt{\frac{2{m^{*}\left( {U_{0} - E} \right)}}{\hslash^{2}}}} \right\}}},} & {{Equation}\mspace{14mu}(3)}\end{matrix}$where, (U₀−E) is barrier height and W is the barrier width. Equation (3)suggests tunneling current at high field gradients may increase inresponse to changes in dimensions of the barrier 118, typicallyreductions in barrier width or barrier height. Even if the physicalthickness of the layer 118 is very large, the “effective” thickness ofthe layer 118 that blocks charge injection may be only severalnanometers, where a 0.1 eV change in barrier height may result in about50 MV/m of change in the critical field and a change of dielectricconstant by an order of magnitude may change the critical field by about100 MV/m. High bandgap materials may form the largest energy barrier 118possible, but still allow for barrier width or the layer thickness thatcomports with use on nano-structures (e.g., CNTs) for the firstelectrode 106. Al₂O₃ between TiO₂ layers, for example, effectivelyblocks leakage current because of its very high bandgap, 8.8 eV, andfavorable resistivity. SiO₂ has an even higher bandgap, 9.2 eV, thanAl₂O₃.

FIG. 3 depicts a plot of Gibbs free energy for various materials,including Al₂O₃, TiO₂, SiO₂. Notably, SiO₂ has also has free energy thatis much more closely aligned with TiO₂ (e.g., within 2% of each other)than Al₂O₃ (as indicated by arrow A). This feature may limit oxygendiffusion between SiO₂/TiO₂ layers (e.g., layers 114, 116) because ofthe similarity of energy of formation. By comparison, the very largedeviation (indicated by arrow B) between Al oxidation with Ti can leadto oxygen vacancies in the TiO₂ layer of an Al₂O₃/TiO₂ laminate. Thesevacancies may promote oxygen diffusion toward Al₂O₃ because of its lowerfree energy for oxidation. This diffusion may be responsible forpositive carriers due to Maxwell-Wagner (MW) relaxation (that isresponsible for high dielectric constant) that increases conductivity ofthe TiO₂ layer and, thus, may result in severe leakage current. To limitthe oxygen diffusion and consequently reduce the leakage current, SiO₂pairs well with TiO₂ as SiO₂/TiO₂ or with Al₂O₃/TiO₂ laminate asSiO₂/Al₂O₃/TiO₂. One benefit of the concepts herein is to increasedielectric constant (k) by virtue of MW relaxation and, in turn,mitigate the resulting higher leakage current by pairing materials withsimilar free energy.

FIG. 4 depicts a schematic diagram of an example of the capacitor 100.This example includes a base 122, for example silicon as the basematerial for the device 102. A supporting layer 124 may reside on thesilicon base 122 to prevent cracking. Niobium (Nb) may prove useful forthis purpose; although materials with similar thermal expansioncoefficient may suffice as well. The first electrode 106 may comprise anarray of ordered structures 126, for example, electrically conductingtubules or CNTs that populate the silicon base 122. The design may alsobenefit from a contact coating 128, like a thin layer of metal (e.g.,platinum (Pt)), which interposes between the tubules 126 and thedielectric laminate 110. This thin layer may resolve manufacturingdefects that can effect conductivity or adhesion of the dielectric tostructures 126 and reduce performance of the device 102. The secondelectrode 108 may form a top contact 130, also made of platinum (Pt) orlike conductive material.

The CNTs 126 may be part of a much larger ordered arrangement (or“array”) that covers the base 122. Generally, it benefits the design tosize the CNTs 126 for the array to optimize total surface area of thecapacitor 100, often in conjunction with steps to optimize deposition ofother material layers, including dielectric and conducting materials.Values for total surface area may correspond with Equation (4) below:

$\begin{matrix}{{{{Total}\mspace{14mu}{Surface}\mspace{14mu}{area}\mspace{14mu}\left( {cm}^{2} \right)} = {\frac{1000 \times \pi\;{dh}}{\left( {d + T} \right)^{2}} + 1}},} & {{Equation}\mspace{14mu}(4)}\end{matrix}$where, d is the diameter of the CNTs 126, T is the total thickness ofthe layers 110, 128, 130 between adjacent CNTs 126, and h is the heightof the CNTs. Equation (4) also includes a “1” to account for aneffective area (or footprint) of the silicon base 122. Optimal surfacearea occurs when spacing D is equal to the diameter d of the CNTs 126.However, aggregate thickness of the layers 110, 128, 130 effectivelyfixes a minimum for the spacing D (and, thus, (d+T) is the smallest unitlength that is repeated in both the x-direction and the y-direction).

FIG. 5 depicts a plot of total surface area as a function of diameter dof the CNTs 126 of FIG. 4 . Data for this plot assumes that thethickness of the dielectric is 50 nm. Also, the data assumes spacing Dis 160 nm (or the lowest possible spacing D given total thickness T)because the effective area is inversely proportional to the spacing D(or directly proportional to the number of tubes 126). The plot showsthat the surface area drops steeply at diameters lower than 160 nm andmaximizes around 200 nm.

FIG. 6 depicts a diagram of manufacturing steps for a method 200 thatresults in an example of the capacitor 100. Generally, this embodimentmay result in high-aspect ratio and isolated CNTs 126 grown in ananodized aluminum oxide (AAO) template by chemical vapor deposition(CVD). AAO templates are particularly useful to synthesize nano-wire andnano-tube arrays by oxidation of aluminum in acid electrolyte. Theresult is a packed array of columnar hexagonal nano-channelsperpendicular to the silicon base 122 that feature uniform pore wallthickness. This approach may also facilitate self-assembled structureswith high aspect ratios, which are difficult or costly to form using aconventional lithographic process.

At stage 202, an Al film is grown on a silicon substrate. An e-beamevaporator may be useful for this purpose. Since the nanostructure ofAAO template affects the orientation and arrangement of CNTs, it is veryimportant to achieve a thermally stable AAO template for hightemperature CVD processes. A temperature between 600° C. to 800° C. maybe required for the CNT growth. This high temperature may putsignificant strain on the thin-films. The thin layer 124 of niobium (Nb)may prevent cracking since the thermal expansion coefficient is close tothat of alumina. Stability analysis of the AAO templates have revealedthat alumina has cracked for substrates without a Nb supporting layer attemperatures from 300° C. to 400° C. The Nb layer 124 also helps makethe bottom of nano-channels more conductive which results in lowerworking voltage for catalyst electrodeposition such as iron (Fe) orcobalt (Co). It is important that the catalyst is only at the bottom ofthe nano-channel to avoid growth of the CNTs 126 from the sidewalls. Athigh temperature, Nb oxidizes to Nb₂O₅, which is insulating. Apre-thermal treatment at 500° C. in 2% H₂ and 98% He reduced theinsulating Nb₂O₅ formed during anodization to semiconducting NbO₂favoring more conductive pathways for uniform electrodeposition.Highly-doped (100) wafers 122 may be used as substrate 104. This stagemay include stages to deposit an aluminum film 132 on the NB layer 124,preferably at a thickness of 100 to 200 μm to allow trenches of 100 to200 μm depth. This stage may occur on one side of the wafer 122, butadditional stages for the other side may benefit the design to allowstructure for a second capacitor to form on the substrate 104.

The stages may include stages to anodize the Al layer 132 in oxalicacid. For example, the wafer 122 may be partially dipped into solution,with aluminum as the anode and a platinum plate as the cathode, untilall of the aluminum is converted into alumina. This process may be doneusing a “two-stage” growth process, which may include both mild and hardionization to form long nano-channels of appropriate length. In oneimplementation, anodization is stopped when approximately half of the ALlayer 132 is consumed. Then the wafer 122 is immersed in a mixture ofchromic acid and phosphoric acid (e.g., at 60° C.) to remove porousalumina formed by the prior stage. This second stage leaves behind thefootprint for the final template. The remaining Al layer 132 will beanodized using, for example, the same conditions as in the second stage.This stage results in an AAO template with better uniformity andimproved nano-channel ordering. The nano-hole diameter can be adjustedby a pore-widening treatment using phosphoric acid. This pore-wideningtreatment may also help to reduce the thickness of any remaining aluminabarrier layer below the bottom tip of the nano-channels, making the basemore conductive for catalyst electrodeposition.

The method 200 can be optimized to achieve desired nano-channel depthsand diameters. Generally, interpore distance may be proportional to theanodization voltage and the pore depth may be proportional to anodizingtime. Processes using high voltage (100-140 V) and higher growth rates(50-100 μmh⁻¹) when compared to mild anodization (MA) at 25 V (2-6μmh⁻¹) may benefit growth of nano-channels, for example, as long as 110μm by a hard anodization technique (HA). For 200 μm long channels, itwill be important to control structural parameters such as pore size,interpore distance, and aspect ratio of nanopores. A protective aluminumoxide layer generated by mild anodization followed by hard anodization(with increase in voltage at a certain rate) has generated highlyordered hexagonal nanochannels.

At stage 204, iron/cobalt (Fe/Co) catalytic nano-particles may beelectrodeposited at the bottom of nano-holes. A three-electrode systemmay be useful for this purpose. This stage may include stages forelectrochemically depositing Co nanoparticles of 100-200 nm by ACelectrolysis in an electrolyte containing CoSO₄, HBO₃ and ascorbic acid.This process may form a cluster of Co nanoparticles with high aspectratio for efficient catalysis. The Co containing AAO templates may beplaced in a tube furnace for reduction (e.g., at 600° C. for 1 hour) ina flow of Ar/H₂ mixture before starting CNT growth. Some embodiments mayalso use Fe as a catalyst, where the process will be the same exceptthat the precursor solution will be iron sulfate.

At stage 206, the CNTs 126 may be grown. Water assisted chemical vapordeposition may be useful for this purpose. The stages may include stagesfor placing the substrate in a quartz tube furnace with Ar and H₂ (e.g.,at 500 sccm and 10 sccm, respectively) to create an oxygen freeenvironment. The furnace is then heated (e.g., to 800° C.). Once thetemperature is maintained, the Ar/H₂ mixture and ethylene gas mixture ispassed through the chamber, which decomposes on the catalyst and createssupersaturation, resulting in CNT growth. Once the required height isobtained, the ethylene flow and heating is stopped while Ar/H₂ flushingis continued. The substrate 122 is removed once it reaches roomtemperature and treated with mild sulfuric acid to etch residual Fe orCo particles.

The thin alumina at the bottom tip of nano-channels helps as a barrierlayer which prevents the Fe/Co nano-particles at high temperature fromdiffusing into the substrate. This helps decomposition of carbonaceousmaterials on the catalyst for efficient CNT growth. The AAO templatemight act as a catalyst media for CNT growth. This may stop the CNTgrowth once it reaches the edge of AAO template. Thus the CNTs will havethe same size and shape as the AAO template.

At stage 208, the method 200 may for etch the aluminum oxide off. Thisstage may expose the bare CNTs 126 to dilute NaOH solution. Any residualFe/Co around the CNTs may be removed, for example, with mild sulfuricacid. Etching of the aluminum oxide can be optimized to make sure thatthe alumina barrier layer at the bottom of the nano-channels is notetched away, thus guaranteeing that the CNTs are intact.

At stage 210, the method 200 may coat the CNTs 126 with the thin layer128 of platinum (Pt). Electroplating may be useful for this purpose,particularly, with the high-aspect ratio of the CNTs, becausetraditional metallization methods (e.g., thermal evaporation,sputtering, etc.) may not fully cover the CNT array. This stage maycorrect certain defects, for example, that may result from “non-perfect”etching of all Al₂O₃ that can leave behind a thin layer of Al₂O₃ at thebottom of the carbon nano-tubes. This thin layer could prove an obstaclefor charge transport, which can limit use of the CNTs 126 as the backelectrodes for the capacitor, as they are conducting material. The Ptlayer 128 may be useful to operate as the back electrode because thismaterial works well with nano-scale coatings like laminate 110.

At stage 212, the method 200 may grow the conformal, laminate 110 on theCNTs 126. Atomic layer deposition (“ALD”) may be useful for thispurpose. This process may avoid “hot spots,” that can occur in chemicaltexturization and rolling processes that practices-to-date employ toincrease surface area of capacitors. These hot spots may reducebreakdown strength. On the other hand, ALD may deposit the oxidelayer(s) uniformly over the CNTs 126 to allow even electric field in alllocations to prevent localized hot spots. The uniformity ofALD-deposited layers is also desirable to avoid defects and lower defectdensity. This feature may avoid trap-assisted tunneling (TAT) ormulti-step trap-assisted tunneling (MTAT), both of which may lead to apercolation event that causes electrical breakdown of the insulator atcritical defect density. Less defects in the oxide layer(s) can alsoavoid Fowler-Nordheim tunneling (or “field assisted thermionicemission”) that can contribute to leakage by allowing tunneling throughthe oxide in response to high electric fields (e.g., >10⁷ V/cm). Theoxide layer(s) from ALD can also avoid Poole-Frenkel effects that occurwhen both defects and high electric fields exist, which can alsocontribute to leakage because the reduction in potential barrier aroundthe trap state in a high electric field allows trapped electrons totunnel out from the trap state.

At stage 214, the method 200 may form the top contact 130.Electroplating may be useful for this purpose. However, this process mayrequire a seed layer with a certain conductivity to achieve uniformdeposits on non-conducting materials such as the dielectricnano-laminate 110. Electroless plating techniques may also apply forohmic contacts to n- and p-doped silicon. In one implementation, thestages may include stages for depositing platinum (Pt) (or another metalor conducting material) as the top contact 130 for the capacitor 100.Electroless deposition of porous platinum on alumina may be carried outusing precursor such as PtCl₂(sty)₂. These films adhere to the substratevery well with tunable sheet resistance of over 5 orders of magnitude.With this process, the Pt nanoparticles assemble themselves into adensely packed, electrically conducting film with nanometer-sized pores,thereby eliminating the need for an additional electrically conductingmatrix material.

FIGS. 7 and 8 depict images of exemplary structure that may result fromuse of the method 200 noted above. FIG. 7 shows an arrangement of theCNTs 126 as uniform, well-separated, isolated structures. Thisarrangement is important to exploit the unique structural and electronicproperties of the CNTs 126. Exemplary process may, for example, growCNTs 126 by the decomposition of ethylene on iron (Fe) or cobalt (Co)catalyst within hexagonal anodized aluminum oxide templates. Thisprocess may allow for controllable and scalable growth of the CNTs 126,as well as a precisely packed structure with controllable spacing of thesame. FIG. 8 depicts a typical SEM cross-section of CNT film grown withFe as a catalyst and a 50 nm Al₂O₃ barrier layer. The CNTs are narrowand about 240 μm in vertical height. The compactness of the CNT filmstems from the thin film of Fe (˜5 nm) on top of the Al₂O₃ barrierlayer. The growth time for the 240 μm tall CNTs was only 2 hrs. Itfollows that uses of the method 200 could, in turn, result in 100 to 200μm tall CNTs, as well.

FIG. 9 depicts a schematic diagram of an example of the device 102. Thisexample includes a pair of capacitors (e.g., a first capacitor 134 and asecond capacitor 136), one each disposed on either side of the siliconbase 122, although other base materials might be used in the process.Coupling mechanism 138 may electrically connect the capacitors 134, 136in parallel. Table 1 below describes functional properties of the device102, where the laminate 110 has a thickness of 50 nm:

TABLE 1 Height Diameter Estimated areal Estimated areal of the of theBreakdown energy density energy density CNTs CNTs Capacitance field(J/cm³) for single- (J/cm³) for double- (μm) (nm) μF/cm² (V/cm) sidedcapacitor sided capacitor 100 50 865 1 × 10⁶ 2162.5 4325 150 50 1297 1 ×10⁶ 3242.5 6485 200 50 1728 1 × 10⁶ 4320 8640 100 50 865 2 × 10⁶ 865017300 150 50 1297 2 × 10⁶ 12970 25940 200 50 1728 2 × 10⁶ 17280 34560

The discussion turns next to another example of the capacitor 100. Asnoted herein, form factors for the ordered structure 126 may vary asnecessary to improve performance of the capacitor 100. These formfactors may include elements that perforate one or more surfaces of thesubstrate 104. Like CNTs, this “nano-perforation” significantlyincreases the surface area of the substrate 104 and can receive theconformal coating of dielectric laminate 110.

FIG. 10 depicts an example of structure to effectuate thisnano-perforated design for the capacitor 100. In this example, theordered structures 126 may embody apertures 140 that penetrate into thebase 122, which may be aluminum. The apertures 140 may populate all orpart of the top (or bottom) of the aluminum base 122. This arrangementmay employ spacing and pitch between adjacent apertures 140 of variousdimensions. However, it may benefit the design (or be subject tomanufacture constraints) to arrange the apertures 140 with uniformspacing in one or more directions along the surface of the substrate104.

FIG. 11 depicts an elevation view of the cross-section of the example ofFIG. 10 . The apertures 140 may embody “blind” holes that terminate at aclosed end 142. But some designs may also benefit from “through” holesthat penetrate the entire thickness of the aluminum base 122 to form apair of open ends. In one implementation, the structure may also includean intermediary contact layer 144 that interposes between the aluminumbase 122 and the dielectric layer 110 that conformally coats the holes140. The top contact layer 130 may reside on the dielectric layer 110.In one example, the top contact layer 130 may comprise material thatfills any interstitial spacing of the hole, as identified by thecross-hatched area enumerated by the numeral 146. The capacitor 100 mayalso include a prep layer 148, like silver paste or other material thatresides on the top contact layer 130. These materials can prepare thedevice for post-process steps (e.g., soldering) or, more generally, toreceive contacts, terminals, or other discrete electrical components.

FIG. 12 depicts a diagram of manufacturing steps for a method 300 thatmay result in an example of the capacitor 100 with apertures 140. Themethod 300 may include, at stage 302, forming the aluminum substrate, atstage 304, forming the apertures 140 and metalizing the same, and atstage 306, preparing the capacitor 100 for post-processes.

At stage 302, the method 300 may affix the Al layer 132 onto the siliconsubstrate. This step may grow or deposit the Al film; however, in oneimplementation, the aluminum layer may alternatively consist of aluminumfoil that adheres (using adhesive) to a surface of the siliconsubstrate. Thickness for the aluminum layer may correspond with adesired depth for the troughs 140, for example, from about 100 μm toabout 350 μm. To increase surface area, the Al layer 132 may be found onmultiple surfaces (e.g., top and bottom) of the silicon substrate.

The stage 302 may also include steps to anodize the Al layer 132 to formthe troughs 140. These steps may provide a well-ordered, uniformarrangement of the troughs 140. In one implementation, the first step ofthe process may require dipping at least part of the silicon substrateinto a bath of 0.3 M oxalic acid at 15° C. at 40V with aluminum as theanode and platinum as the cathode. The substrate may remain for a perioduntil half of the aluminum converts into alumina. The second step of theprocess may immerse the partially-anodized aluminum-on-silicon substrateinto a bath of chromic acid and phosphoric acid (at 60° C.). This bathcan remove any porous alumina that forms during the first step. Theprocess may also repeat the first step to anodize any remainingaluminum. In one implementation, the process may include steps to adjustdimensions for the troughs 140, for example, by treating the anodizedsilicon substrate in 5 wt % phosphoric acid.

At stage 304, the method 300 may deposit various layers into theapertures 140. This stage may include steps to add the intermediary or“back” contact layer 144, typically Cu deposited using electroplating.The stage also includes steps to add the conformal laminate 110. Asnoted above, atomic layer deposition (ALD) may prove beneficial for thispurpose, but other processes may work as well, for example, sputtering,chemical vapor deposition (CVD), molecular beam epitaxy (MBE), etc. Thesteps may deposit porous platinum (Pt) as the top contact layer 146, forexample, by electroless electroplating. This process may convert[PtCl₂(tly)2] precursor in toluene at 80° C. into a nanoporous,electrically conductive Pt film on the dielectric laminate 110.Alternatively, ALD techniques may deposit metal, metal-like ortransparent conductor material to form the top contact layer 146.

At stage 306, the method 300 may deposit solder paste onto the device.This stage may also include other post-processing steps, for example,steps to attach peripheral or discrete electronics devices to thecapacitor 100. These steps may prepare the device for additionalpackaging operations, like overmolding or mounting one or more of thecapacitor 100 into or as part of a bigger assembly.

In light of the foregoing discussion, the improvements herein result inultra-high capacitance solid state capacitors that may reliably replacebattery storage. These devices include very high surface area electrodesin a small footprint. These electrodes may comprise nano-structures,like nano-tubes or nano-holes. A specialized dielectric resides on thestructures and between the electrodes. This dielectric may embody aconformal coating or laminate with material layers, each made ofmaterials with very high dielectric constant and dielectric strength.Thickness of this laminate balances any tradeoff between capacitance andbreakdown voltage.

Manufacture of the embodiments herein may include steps for formingcarbon nano-tubes or nano-holes on a substrate; conformally coating thecarbon nano-tubes or nano-holes with a dielectric laminate; metalizingthe carbon nano-tubes or nano-holes with conducting material thatinterposes between the carbon nano-tubes or nano-holes and thedielectric laminate; metalizing the dielectric laminate; depositingaluminum on the substrate; etching the aluminum to form a template forthe carbon nano-tubes; and depositing aluminum oxide in the template.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. An element or functionrecited in the singular and proceeded with the word “a” or “an” shouldbe understood as not excluding plural said elements or functions, unlesssuch exclusion is explicitly recited. References to “one embodiment” ofthe claimed invention should not be interpreted as excluding theexistence of additional embodiments that also incorporate the recitedfeatures. Furthermore, the claims are but some examples that define thepatentable scope of the invention. This scope may include andcontemplate other examples that occur to those skilled in the art. Suchother examples are intended to be within the scope of the claims if theyhave structural elements that do not differ from the literal language ofthe claims, or if they include equivalent structural elements withinsubstantial differences from the literal language of the claims.

Examples appear below that include certain elements or clauses one ormore of which may be combined with other elements and clauses describeembodiments contemplated within the scope and spirit of this disclosure.

What is claimed is:
 1. A capacitor, comprising: a base material; carbonnano-tubes disposed on the base material, each forming a body thatextends from the base material; a metal layer disposed on the body ofthe carbon nano-tubes; a non-electrolytic conformal coating disposed onthe metal layer to cover a majority of the body, the conformal coatingfilling space between adjacent carbon nano-tubes, the non-electrolyticconformal coating comprising: a first layer of material disposed on themetal layer, the first layer comprising alternating layers of aninsulating material and a semiconductor material; and a second layer ofmaterial disposed on a last one of the alternating layers of the firstlayer, the second layer comprising an insulating material; and aconductive material disposed on the non-electrolytic conformal coating,the conductive material filling all remaining space not filled by thenon-electrolytic conformal coating between adjacent carbon nano-tubes.2. The capacitor of claim 1, wherein the conformal coating comprises afirst coating and a second coating, each comprising the first layer ofmaterial and the second layer of material.
 3. The capacitor of claim 1,wherein the conformal coating comprises a first coating and a secondcoating, only one of which comprises the first layer of material and thesecond layer of material.
 4. The capacitor of claim 1, wherein the anon-electrolytic conformal coating comprises a third layer of materialdisposed on the second layer of semi-conductive material and that isdifferent from the second layer of semi-conductive material.
 5. Thecapacitor of claim 1, wherein the a non-electrolytic conformal coatingcomprises a third layer of insulating material disposed on the secondlayer of semi-conductive material.
 6. The capacitor of claim 1, whereinthe first layer of insulating material and the second layer ofsemi-conductive material have Gibbs free energy within 2% of each other.7. A capacitor, comprising: a base material; nano-apertures having anopening on at least one surface of the base material; a metal layerdisposed in the nano-apertures so as to cover the base material in thenano-apertures; a non-electrolytic conformal coating disposed in thenano-aperture, the non-electrolytic conformal coating comprising: afirst layer of material disposed on the metal layer, the first layercomprising alternating layers of an insulating material and asemiconducting material; and a second layer of material disposed on alast one of the alternating layers of the first layer, the second layercomprising an insulating material; and a conductive material disposed inthe nano-aperture, the conductive material filling all space not filledby the non-electrolytic conformal coating in the aperture and alsocovering the opening.
 8. The capacitor of claim 7, wherein the firstlayer of insulating material and the second layer of semi-conductivematerial have Gibbs free energy within 2% of each other.
 9. The deviceof claim 7, wherein the a non-electrolytic conformal coating comprises athird layer of insulating material disposed on the second layer ofsemi-conductive material.